Advancing technology is constantly demanding strong, stiff and light metals. A metal fulfilling these requirements is beryllium and beryllium alloys which are lighter than aluminum and more rigid than steel. However, beryllium is not widely used because by metallic standards it is brittle. The atomic structure is such that in the polycrystalline form its atoms cannot slip in enough directions to permit the individual metallic grains to deform without cracking. This characteristic of beryllium makes it advantageous properties less attractive since once a small crack is initiated, it can grow easily and cause failure of the entire component. It would be of great benefit to fabricators and designers if beryllium's tendency to failure at a low strain were modified so that uniform strains of 5 - 10% could be applied before failure. This would bring beryllium's strain to fracture into line with that of other high strength engineering materials such as low alloy steels, titanium and aluminum alloys. The brittleness of beryllium at room temperature has been recognized since 1937 (see, W. Kroll, Metals and Alloys, Vol. 8, 1937, p. 349) and between then and the present time, numerous research programs have been undertaken with the object of improving ductility.
These programs have met with little success but results will be briefly described since in some cases they involve techniques which are closely related to those described in the instant application.
The relevant techniques in the prior art are purification, grain refinement and heat treatment. In the prior work on purification extremely pure (99.999%) beryllium has been produced by zone refining in an effort to remove dissolved elements that might be embrittling the metal grains. This purification did improve ductility in certain orientations in single crystals but in normal polycrystalline beryllium, ductility was virtually unchanged. In the present invention, high purity is stressed, not because impurities might embrittle the grains as in the prior work, but to avoid the formation of a liquid phase which will coarsen the beryllium oxide particles.
The prior work on grain refinement of beryllium has involved cold working and recrystallization of both castings and hot pressed block and the hot pressing of ultrafine powders. Cold working and recrystallization have the disadvantage of producing a material with a high degree of crystallographic texture and anisotropic mechanical properties. The use of ultrafine powders, while producing a small initial grain size, does so at the expense of an increased oxide content which, as will be subsequently discussed, reduces ductility. In addition, applicants have found that fine grain structures produced by any means will not be stable during hot pressing or high temperature annealing treatments unless the median grain boundary beryllium oxide size is maintained below critical sizes.
The use of heat treatment to improve the room temperature ductility of beryllium has been largely unsuccessful. Some improvement in elevated temperature ductility in relatively impure materials has been achieved by aging at temperatures less than 1450.degree. F to form a ternary compound AlFeBe (see, J. A. Carrabine et al, AIME Met. Society Conferences, Vol. 33. Beryllium Technology, Vol. 1, 1966, p. 239 ). A stress relieving treatment less than 1500.degree. F is normally given to beryllium block to remove residual cold work produced by thermal stresses after pressing. Stress relieving treatments above 1500.degree. F are not normally given to conventional hot pressed beryllium block. It has not previously been recognized by the art that annealing treatments above 2000.degree. F are necessary and desirable in block processed to contain a finer than normal oxide particle size. This effect is a result of the higher recrystallization temperature of block containing fine particles.